cellForItemAt.swift

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

  // Working with functions categories collection view
  if collectionView == self.functionsCategoriesCollectionView {
      let cellFunctionsCategories = collectionView.dequeueReusableCell(withReuseIdentifier: "cellCat", for: indexPath) as! DevicePageFunctionsCategoriesCVCell
      cellFunctionsCategories.cellTitleContent.text = ICDatabase.objects(FunctionCategory.self)[indexPath.row].name.uppercased()
      
      cellFunctionsCategories.setGradientBorder(width: 10, colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0), UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0)])
      
      if indexPath == cellSelectionIndexPath {
          if let cellToDeselect = collectionView.cellForItem(at: self.cellSelectionIndexPath) as? DevicePageFunctionsCategoriesCVCell {
              
              cellToDeselect.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.clear.cgColor], angle: 0)
              
              // Get all functions
              self.filtered = GlobalVariables.currentProduct.functions.filter { _ in
                  return true
              }
              
              self.functionsCollectionView.reloadData()
          }
      }
      return cellFunctionsCategories
  }
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    
    // Working with functions categories collection view
    if collectionView == self.functionsCategoriesCollectionView {
        
        let cell = collectionView.cellForItem(at: indexPath) as! DevicePageFunctionsCategoriesCVCell
            cell.isHidden = false
            cell.cellView.isHidden = false
            cell.isSelected = true
        
            cell.cellView.clipsToBounds = true
            cell.cellView.layer.cornerRadius = 25
            cell.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)
        
        if cellSelectionIndexPath == indexPath {
            // it was already selected
            cellSelectionIndexPath = nil
            collectionView.deselectItem(at: indexPath, animated: true)
            cell.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.clear.cgColor], angle: 0)
            
            self.filtered = GlobalVariables.currentProduct.functions.filter { _ in
                return true
            }
            
            self.functionsCollectionView.reloadData()
            
        } else {
            // wasn't yet selected, so let's remember it
            cellSelectionIndexPath = indexPath
            
            // Filter with seletec category name
            let cellCategoryName = ICDatabase.objects(FunctionCategory.self)[indexPath.row]
            
            self.filtered = GlobalVariables.currentProduct.functions.filter { function in
                return function.functionCategory.contains(cellCategoryName)
            }
            
            self.functionsCollectionView.reloadData()
        }
        
    }
}

 if indexPath == cellSelectionIndexPath {
      if let cellToDeselect = collectionView.cellForItem(at: self.cellSelectionIndexPath) as? DevicePageFunctionsCategoriesCVCell {
          
          cellToDeselect.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.clear.cgColor], angle: 0)
          
          // Get all functions
          self.filtered = GlobalVariables.currentProduct.functions.filter { _ in
              return true
          }
          
          self.functionsCollectionView.reloadData()
      }
  }

Code mentioned above is bad approach. You should not call reloadData inside cellforRowAt method. Inside if block, if condition is true, update same cell as shown below.

if indexPath == cellSelectionIndexPath {
      cellFunctionsCategories.setGradientBorder(width: 10, colors: [UIColor.green, UIColor.blue])
}

didSelect method is just to capture selectedIndexPath and call reloadData. Don't update cell there directly. Follow my code added in stack overflow, those 2 lines are enough for this requirement.

Thanks a lot for your support I try this now

I edited my code as below. But I do not understand what I'm making wrong :

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        // Working with functions categories collection view
        if collectionView == self.functionsCategoriesCollectionView {
            let cellFunctionsCategories = collectionView.dequeueReusableCell(withReuseIdentifier: "cellCat", for: indexPath) as! DevicePageFunctionsCategoriesCVCell
            cellFunctionsCategories.cellTitleContent.text = ICDatabase.objects(FunctionCategory.self)[indexPath.row].name.uppercased()
            
            cellFunctionsCategories.setGradientBorder(width: 10, colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0), UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0)])

            //when cell selected add gradient to fill it
            if indexPath == cellSelectionIndexPath {
                cellFunctionsCategories.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)
            }
            
            return cellFunctionsCategories
        }

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
        // Working with functions categories collection view
        if collectionView == self.functionsCategoriesCollectionView {
            
            if cellSelectionIndexPath == indexPath {
                // it was already selected
                cellSelectionIndexPath = nil
                collectionView.deselectItem(at: indexPath, animated: true)
                
                let selectedItems = collectionView.indexPathsForVisibleItems
                for indexPath in selectedItems {
                    collectionView.deselectItem(at: indexPath, animated:true)
                    if let cell = collectionView.cellForItem(at: indexPath) as? DevicePageFunctionsCategoriesCVCell {
                        
                        cell.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.clear.cgColor], angle: 0)
                    }
                }
                
                self.filtered = GlobalVariables.currentProduct.functions.filter { _ in
                    return true
                }
                
                self.functionsCollectionView.reloadData()
                
            } else {
                // wasn't yet selected, so let's remember it
                cellSelectionIndexPath = indexPath
                
                if let cell = collectionView.cellForItem(at: indexPath) as? DevicePageFunctionsCategoriesCVCell {
                    
                    cell.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)
                }
                
                // Filter with seletec category name
                let cellCategoryName = ICDatabase.objects(FunctionCategory.self)[indexPath.row]
                
                self.filtered = GlobalVariables.currentProduct.functions.filter { function in
                    return function.functionCategory.contains(cellCategoryName)
                }
                
                self.functionsCollectionView.reloadData()
            }
            
        }

Replace didSelectItem method with below code only. This will store selected indexPath, changing color is handled by cellForRowAtIndex method. You just need to reload collectionView.

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    
        // Working with functions categories collection view
        if collectionView == self.functionsCategoriesCollectionView {
              cellSelectionIndexPath = indexPath
              collectionView.reloadData()
        }
}

I edited my code like you proposed. Here is the result : http://digitalblend.fr/BUG.mov I do not understand why all cells background change...

1. cellFunctionsCategories.setGradientBorder(width: 10, colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0), UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0)])

2. cellFunctionsCategories.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)

Does these two methods does same action with different color?

The first add a gradient color only for the cell's border and second one a gradient color for the cell's background

Put this in CellForRowAtIndex method

    // you have to set default color ( this must be your deselected color)
    cellFunctionsCategories.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.white.cgColor], angle: 45)

        //when cell selected add gradient to fill it
        if indexPath == cellSelectionIndexPath {
              // (this must be your selected color)
              cellFunctionsCategories.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45) 
        }

I have exactly the same problem with :

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
        // Working with functions categories collection view
        if collectionView == self.functionsCategoriesCollectionView {
            cellSelectionIndexPath = indexPath
            
            let cellFunctionsCategories = collectionView.dequeueReusableCell(withReuseIdentifier: "cellCat", for: indexPath) as! DevicePageFunctionsCategoriesCVCell
            
                cellFunctionsCategories.cellView.addGradiant(colors: [UIColor.clear.cgColor, UIColor.white.cgColor], angle: 45)
            
            //when cell selected add gradient to fill it
            if indexPath == cellSelectionIndexPath {
                // (this must be your selected color)
                cellFunctionsCategories.cellView.addGradiant(colors: [UIColor(red: 127.0/255.0, green: 127.0/255.0, blue: 127.0/255.0, alpha: 1.0).cgColor, UIColor(red: 47.0/255.0, green: 47.0/255.0, blue: 47.0/255.0, alpha: 1.0).cgColor], angle: 45)
            }
            
            collectionView.reloadData()
        }
}